The speaker efficiency won't be an issue. If you have amps with VERY high input sensitivity - i.e. much less than 1V to reach full volume, then you might want the total gain attenuated somewhat, but most of our customers are perfectly happy with the 15dB in the 9.0SE.
If you are looking at the 9.0, I would strongly urge you to consider the 9.0 'Signature Edition' with tube-rectified PS. It REALLY is that much better. Please contact us for the location of your nearest ModWright dealer.
Thanks,
Dan